Field of the Invention
The aspect of the embodiment relates to an image processing apparatus, a control method of the image processing apparatus, and a storage medium, and particularly relates to gradation conversion processing of a moving image signal.
Description of the Related Art
In a monitoring system, a monitoring camera distributes an entire image through a network, so that a user can monitor the distributed image. Further, there is also provided a monitoring system which enables a user to monitor a part of an image cut out (cropped) from the distributed image. The user can freely specify a region to be cropped. At this time, if the distributed image presents a scene with a great luminance difference, overexposure or underexposure may occur in the image monitored by the user.
Conventionally, as a method of executing gradation conversion processing for solving the above-described issue, there has been known high dynamic range processing (hereinafter, called as “HDR processing”) for acquiring a high dynamic range image by combining a plurality of images obtained in different exposure conditions. However, as a difference in imaging timing between the plurality of images arises, this method is thought to be unsuitable for capturing a scene including moving objects. Further, there may be an issue in that a luminance difference occurs in the image according to a position of the moving object, and an issue in that a double image is generated because of failure in image composition. Furthermore, there is also an issue in that a frame rate decreases because image composition is executed by acquiring a plurality of images.
With respect to the above-described issues, Japanese Patent Application Laid-Open No. 03-126377 discusses a method for executing gradation conversion by generating a gamma curve such that overexposure or underexposure can be suppressed in a luminance region with high frequencies, based on a frequency of each luminance in a cumulative histogram of one frame. The gamma conversion processing discussed in Japanese Patent Application Laid-Open No. 03-126377 is an image processing method to be executed through processing in one frame according to a characteristic of a luminance histogram of an input image, and gradation conversion can be executed without losing visibility of a moving object as it does not require composition of a plurality of images.
However, through a conventional method of generating a gamma curve from the cumulative histogram as discussed in Japanese Patent Application Laid-Open No. 03-126377, a gradation range near the gradation value having the high frequency is improved. Therefore, in the above-described use case of the monitoring camera, there might be an issue in which the user cannot see the image with appropriate gradation when a plurality of users monitors the images. For example, even if a user desires to see the image (image being cropped by the user), i.e., a gradation characteristic of the gradation value is to be improved, it is not possible to improve the gradation characteristic if a frequency of the gradation value is less than a frequency of a gradation value of the image being cropped by another user.
Therefore, a method of improving gradation characteristics of a plurality of images (crop images) even if a plurality of users individually performs cropping (cutting) on a distributed image is sought.